Another issue to remember: unlike most rifles, headspace in an AR occurs between the bolt and the barrel extension (which is normally considered an integral part of the barrel, as they are rarely ever disassembled or replaced).  You can check headspace with just a barrel assembly and bolt, and installing the barrel onto an upper receiver will NOT change the headspace at all.

You can ream out a short chamber with a reamer (unless the chamber has been chrome-lined), but the only way (outside of a metal shop and an experienced AR 'smith) to correct for excessive headspace is to find a bolt on the long end of its tolerance and hope that's enough.

Military-accepted parts are tested against their own tolerances individually, with tight enough specs that any bolt will correctly headspace with any rifle.  They are checked again anyway once the rifle has been assembled.

Non-military parts may or may not be checked the same way (in fact, it is KNOWN that few of the "parts house" components are checked this way), so you can't assume that what is safe (if not wise) to do in the military is also safe to do with civilian parts.

To remove as much doubt as possible, stick with Colt or Bushmaster parts.  Otherwise, check headspace or risk your face and body.  (And I recommend checking it with Colt and Bushy parts too!)

-Troy